Tuberculosis (TB) is one of the most prevalent infectious diseases in the modern world. Roughly 2 billion people are infected with Mycobacterium tuberculosis (Mtb), the causative agent of TB, which leads to approximately 2 million deaths annually. One of the major hurdles to effective prevention and treatment is the complex etiology of TB. It is largely unclear what combination of factors determine whether an individual contains the infection in a relatively stable, asymptomatic state, or progresses to active infection, where the immune system does not maintain control of Mtb and there is a high incidence of lung necrosis and pathology. Fortunately, some of the important immune players in TB have been identified: specifically, CD4+ T-cells are necessary to control infection, while neutrophil influx into the pulmonary compartment correlates with hyper- inflammation and active disease. These two contrasting outcomes invite the question of what happens in the bacterial environment to facilitate a switch from immune-controlled quiescence to dysregulated immune- pathology and active bacterial replication. Importantly, this question cannot be adequately addressed by simply observing immune responses. Bacterial environment must also be analyzed to understand the pathogenic reaction to the host response. Therefore, the proposed studies will query host-pathogen interactions from a combined perspective, both that of the host and the bacteria, to provide a comprehensive view of contained versus active Mtb infection. To accomplish this, we will take a directed approach based on the data from a large scale genetic screen of Mtb fitness in multiple mouse models of infection. These initial data suggest that neutrophil-mediated immune regulation may dampen functional T-cell suppression of Mtb. Moreover, this screen provided clues as to some of the important bacterial functions for Mtb survival in differential (inflammatory verses non-inflammatory) host environments. Consequently, we will probe host- pathogen interactions using a parallel approach: we will use mouse models of distinct types of infection to elucidate the host neutrophil and T-cell parameters that facilitate contained infection or active disease, we will utilize TNseq-based fitness profiling to define the bacterial genetic components that are impacted by a neutrophil-mediated immune response, and we will use bacterial mutagenesis and pre-defined mouse models of inflammation to identify the specific bacterial functions that are important in these different disease states. Thus, these experiments are designed to expand upon our preliminary findings while providing a novel and balanced analysis of host-pathogen interactions during TB. By uncovering those critical points that facilitate active TB disease, we can advance rational drug and vaccine design for improved treatment and prevention.